A Reaction Between High Mn-High Al Steel and CaO-SiO₂-Type Molten Mold Flux: Part II. Reaction Mechanism, Interface Morphology, and Al₂O₃ Accumulation in Molten Mold Flux

Following a series of laboratory-scale experiments, the mechanism of a chemical reaction \(4[\rm{Al}] + 3(\rm{SiO}_2) = 3[\rm{Si}] + 2(\rm{Al}_2\rm{O}_3)\) between high-alloyed TWIP (TWin-Induced Plasticity) steel containing Mn and Al and molten mold flux composed mainly of CaO-SiO₂ during the continuous casting process is discussed in the present article in the context of kinetic analysis, morphological evolution at the reaction interface. By the kinetic analysis using a two-film theory, a rate-controlling step of the chemical reaction at the interface between the molten steel and the molten flux is found to be mass transport of Al in a boundary layer of the molten steel, as long as the molten steel and the molten flux phases are concerned. Mass transfer coefficient of the Al in the boundary layer (\(k_{\rm{Al}}\)) is estimated to be 0.9 to 1.2 × 10⁻⁴ m/s at 1773 K (\(1500\,^{\circ}\)C). By utilizing experimental data at various temperatures, the following equation is obtained for the \(k_{\rm{Al}}; \ln k_{\rm{Al}} = -14,290/T - 1.1107.\) Activation energy for the mass transfer of Al in the boundary layer is 119 kJ/mol, which is close to a value of activation energy for mass transfer in metal phase. The composition evolution of Al in the molten steel was well explained by the mechanism of Al mass transfer. On the other hand, when the concentration of Al in the steel was high, a significant deviation of the composition evolution of Al in the molten steel was observed. By observing reaction interface between the molten steel and the molten flux, it is thought that the chemical reaction controlled by the mass transfer of Al seemed to be disturbed by formation of a solid product layer of MgAl₂O₄. A model based on a dynamic mass balance and the reaction mechanism of mass transfer of Al in the boundary layer for the low Al steel was developed to predict (pct Al₂O₃) accumulation rate in the molten mold flux.